Battery system

ABSTRACT

The present invention relates to a cell for the production of electric energy by reaction between hydrogen peroxide or oxygen, and aluminium or lithium or a mixture thereof, and hydroxyl ions in water, where the cathodes are cylindrical and based on radially oriented carbon fibres attached to a stem of metal. The novel feature of the invention is that by utilizing such cathodes in a cell with circulating electrolyte, it is possible to keep the concentration of oxidant in the electrolyte low and thus obtain high utilization of the reactants.

FIELD OF THE INVENTION

The invention relates to a battery for the production of electric energyby reaction between hydrogen peroxide (H₂O₂) or oxygen, and aluminium orlithium or a mixture thereof, and which utilizes electrodes of bottlebrush shape, and which can mechanically be charged by inserting metalanodes. The advantages of the invention are a battery with highutilization of the reactants combined with the possibility of quickmechanical charging.

The main object of the invention is a battery which can be utilized forthe energy supply of small unmanned underwater vehicles (UUV), but theinvention is not limited for this use only. The battery is going to havepressure-compensated operation, i.e. the battery does not have to beencapsulated in a pressure tank. The cells are based on an alkalineelectrolyte with H₂O₂ or oxygen as the oxidant, and metal anodes. Theoxidant is added to the electrolyte, and the electrolyte is pumpedthrough the cells in the battery. The anode material are alloys whichform soluble reaction products by anodic dissolution in alkalineelectrolyte.

RELATED PRIOR ART

“Bottle brush” electrodes are known from NO 171 937, (Garshol andHasvold), where there is described electrodes formed as bottle brushes.The purpose of these bottle brush electrodes are to obtain maximum areaof the electrodes combined with good conductivity, low resistance toflow and a sufficiently mechanically stable solution.

Batteries which utilize oxygen or hydrogen peroxide, where a circulationof electrolyte takes place, are known from U.S. Pat. No. 4,305,999(Zaromb). The anode is made of consumable metal, especially zinc,magnesium or aluminium. The purpose of the '999 Patent is to regulatethe electrolyte level in the battery cell in relation to the powerconsumption in such a way that unnecessary corrosion is prevented.

U.S. Pat. No. 4,910,102 describes a battery and a process for operatingthe battery, where bipolar electrodes are included consisting of aninert cathode which works as a hydrogen peroxide electrode, and an anodeplate of aluminium, magnesium or alloys thereof. (In the abstract forU.S. Pat. No. 4,910,102 there seems to be an error: There is referred toa hydrogen electrode, but it seems to be a hydrogen peroxide electrode.Further there is referred to bipolar cathodes; the correct term seems tobe bipolar electrodes). The electrolyte flows through the battery, andH₂O₂ is added in concentrations between 0.5% and 30% as volume part ofthe electrolyte. The electrolyte is, for example, sea water.

Expected time of discharge for such a battery for an unmanned underwatervehicle is long, typically more than 10 hours. The long time ofdischarge gives low current densities which subsequently allows forrelatively large electrodes and a large distance between the electrodesin the cells of the battery. The battery comprises one or more cells.Anode materials of current interest are alloys which form solublereaction products by anodic dissolution in an alkaline electrolyte. Therate of corrosion of the metal in the electrolyte has to be relativelylow, which excludes the alkaline metals, except for lithium. Mostappropriate are probably alloys of aluminium such as utilized earlier inFFI's alkaline aluminium/air battery and as described in Hasvold:“Development of an alkaline aluminium/air battery system”. Chemistry andIndustry (1988), pp 85-88, and Størkersen: “Development of a 120 W/24VMechanically Rechargeable Aluminium-Air Battery for MilitaryApplications”. Power Sources 13, (1991), Ed.: Keily, T. and Baxter, B.W., pp 213-224.

Galvanic cells, which utilize hydrogen peroxide (HP) as oxidant(“cathodic depolarizer”), have been known for long. In some systems, HPis utilized directly in the cell, while in other systems, HP is used asa storage medium for oxygen, i.e. as an oxygen carrier. In the lastcase, one lets H₂O₂ decompose in a reactor and supplies the cells withoxygen from this reactor:

2H₂O₂=2H₂O+O₂  (1)

The oxygen is consumed in a gas cathode in a fuel cell or in ametal/oxygen battery. A typical example of such a technology isAlupowers's alkaline aluminium/oxygen battery for operation of unmannedunderwater vehicles and is described in Deuchars, G. D. et al.:“Aluminium- hydrogen peroxide power system for an unmanned underwatervehicle” Oceans 93 (1992), Vancouver, pp 158-165. In other cells, ase.g. described by Zaromb in U.S. Pat. No. 4,198,475, HP is addeddirectly to the cathode in an aluminium/hydrogen peroxide battery withalkaline electrolyte. Whether HP decomposes in the electrolyte underformation of oxygen which in turn is reduced on the cathodes, or HP isreduced directly on the electrode surface, makes little difference inpractice.

The advantage of utilizing HP as oxidant instead of oxygen is that thestorage is substantially easier. Further, HP is miscible with water andcan be added directly to the electrolyte in the desired concentration.In a UUV, the storage can also, if desired, be made outside the pressurehull. According to equation (1), 1 kg pure HP equals 0.471 kg oxygen.Pure HP implies a handling risk, as HP is unstable and the decompositionof HP releases a considerable amount of energy. This risk isconsiderably reduced by increasing the contents of water. 70% HP can behandled by attention to special precautionary measures, and at 50%, theheat of decomposition is no longer sufficient for complete vapourizationof the water forming. In 70% HP, the “oxygen part” of the weightconstitutes approx. 33% and in 50% HP approx. 24%. Liquid oxygen, LOX,provides effective storage based on weight, but cryogenic storage ofoxygen demands a certain thickness of the isolation, so that one forsmall systems gets very voluminous tanks in relation to the usefulvolume. The demand for insulation increases with the time the oxygen isto be stored. Further, a cryogenic storage tank is basically notsuitable at large external pressures. For this reason, the storage in aUUV has in practice to be carried out in a pressure tank, which makesthe system not very well suited for application in small vehicles.

The last and most common storage form for oxygen is under pressure incylindrical- or spherical pressure tanks (bottles). This is verypractical as long as the pressure of the battery is less than the bottlepressure, as the oxygen supply can be regulated by operating a valve.The tanks can be exposed to external pressure and be arranged on theoutside of the pressure hull. Traditional metal tanks are heavy,typically an empty weight of 15 kg can store 4 kg oxygen, but fibrereinforced tanks can be made considerably lighter, and a storagecapacity of 40-50% is not unlikely in the future.

At 300 bar, oxygen has a density of approx. 0.4 kg/litre and a systemdensity by utilizing fibre-reinforced tanks of approx. 0.2 kgoxygen/litre. This provides somewhat more voluminous storage byutilizing pressure tanks than by oxygen storage in the form of 50% HP.Further, it has to be taken into consideration that with UUV batterieswhich operate at ambient pressure, the oxygen has to actively be pumpedout of the bottles when external pressure is higher than the bottlepressure. This is a considerable problem for UUV's which are to operateat great depths. In comparison, a HP storage will normally have ambientpressure independent of depth. Finally, it should be mentioned thatwhile HP can directly be mixed in the electrolyte in the desiredconcentration, the solubility of oxygen in the electrolyte is low. Evenif the solubility increases proportionally with pressure, the rate ofdissolution is relatively slow, which can entail a complex system forthe mixing of oxygen into the electrolyte. For the above mentionedreasons, one has primarily considered the use of HP in UUV batteries,but an oxygen-based battery which operates at a pressure of more than 5to 10 bar, will have almost identical properties.

A problem with batteries where the oxidant is dissolved in theelectrolyte is that the oxidant and anode metal can be consumed bydirect reaction with each other. For Al this gives:

2Al+3H₂O₂+2OH⁻=2Al(OH)₄ ⁻  (2)

2Al+3/2O₂+2OH⁻=2Al(OH)₄ ⁻  (3)

These non-current-producing reactions lead to losses of reactants andsubsequently to reduced energy output from the cell. Further, they leadto an undesired heat generation in the cell. Reactions (2) and (3) alsolead to a rise in anode potential, which reduces the cell voltage.

Both oxygen and H₂O₂ are strong oxidants which in strongly alkalinesolutions very quickly react with anode materials of current interest,such as aluminium/tin alloys. By sufficiently high reactivity, the rateof reaction will be limited by the transport of oxidant to the anodesurface (limiting current conditions). The rate of transport is given bythe local hydrodynamic conditions. Hydrodynamic parameters whichinfluence the limiting current are among others the character of theelectrolyte flow (laminar/turbulent) and local velocity of flow and thephysical dimensions of the anode. The rate of transport at limitingcurrent will be close to being proportional to the concentration ofoxidant. Thus, it is important to keep the concentration of oxidant inthe electrolyte as low as possible. To reduce this parasitic reactionbetween oxidant and anode metal, it is common to use a membrane whichseparates the solution which surrounds the cathodes and which containsthe oxidant (the catholyte) from the electrolyte surrounding the anodes(the anolyte). The loss according to (2) and (3) will then be reduced tothe amount of oxidant which diffuses through the membrane.

FIG. 1 shows schematically a cell based on separate anolyte andcatholyte, according to the related prior art.

The cell is composed of an anode chamber I with an anode II in ananolyte III. Between the anode chamber and the cathode chamber V, amembrane or separator IV which separates the anolyte III from thecatholyte VII, but which allows transport of current (ions) through themembrane. The positive electrode in the cell, the cathode VI is made ofan electrically conductive material which is a good catalyst, or iscovered by a good catalyst, with a view to the reduction of hydrogenperoxide or oxygen or which catalyzes the decomposition of hydrogenperoxide followed by the reduction of oxygen.

The anode II can for example consist of pure aluminium alloyed with 0.1%tin, while the anolyte III can e.g. be 7 M KOH or NaOH. Also, thecatholyte VII can consist of 7 M KOH, but will also contain oxidant(H₂O₂ or O₂). For ensuring good stirring and thermal control, theelectrolytes may be circulated between one or more cells and areservoir. Further, the oxidant has to be supplied to the catholyte asit is consumed. The addition can be controlled by either a sensor on theoxidant in the electrolyte or from the calculated consumption fromFaraday's law and an estimate of the expected corrosion.

The cell according to the related art in FIG. 1 produces current byoxidation of aluminium at the anode II according to

2Al+8OH⁻=2Al(OH)₄ ⁻+6e⁻  (4)

The electrons are consumed at the cathode VI according to

3H₂O₂+6e⁻=6OH⁻  (5)

either by direct reduction of HP, or by decomposition followed by thereduction of oxygen forming: $\begin{matrix}{{{3H_{2}O_{2}} = {{\frac{3}{2}O_{2}} + {3H_{2}O}}}{{{\frac{3}{2}O_{2}} + {3H_{2}O} + {6e^{-}}} = {6{OH}^{-}}}} & \text{5b}\end{matrix}$

The sum of the anode and cathode reactions gives the cell reaction, i.e.

2Al+3H₂O₂+2OH⁻=2Al(OH)₄ ⁻  (6)

It appears from the equations that there is a consumption of bothaluminium, hydrogen peroxide and hydroxyl ions when the cell deliverscurrent. According to equation (6) and Faraday's law, 9 grams of Al, 17g HP and 62 g 7 M KOH are consumed to deliver 1 F current. Thiscorresponds to 305 Ah/kg.

Unfortunately, in a real system, a substantially lower charge density isobtained. This is partly due to the fact that the entire alkalineelectrolyte cannot be consumed, and partly due to parasitic reactionswhich lead to the consumption of reactants (corrosion reactions). Besidedirect reactions between oxidant and aluminium according to equations(2) and (3), there is a reaction between aluminium and water under thegeneration of hydrogen:

2Al+3H₂O+2OH⁻=2Al(OH)₄ ⁻+3H₂  (7)

Also by this reaction there is a consumption of hydroxyl ions. Inaddition, this reaction leads, unlike the reactions (2) and (3), to theformation of hydrogen. Hydrogen which is not dissolved in theelectrolyte, will form gas which has to be removed from the battery asit is formed. This is unlike oxygen, which can be removed by cathodicreduction in the battery as long as it is dissolved in the electrolyte.Especially at high pressures where the solubility of oxygen in theelectrolyte is high, the oxygen will not contribute to the formation ofgas in the battery. Notwithstanding this, an aluminium-based batterywith an alkaline electrolyte, capable of functioning, has to have asystem for the handling of gas being formed.

Generally, corrosion does not lead to reduced energy density until thecorrosion current approaches the same order of magnitude as the averagecell current. Further, it is evident that it is desired to keep thelosses due to corrosion low, both for preventing loss of reactants, andalso for reducing the problem with the formation of hydrogen in thebattery.

The decomposition of water according to (7) can be kept at a low levelby utilizing electrolytes and alloys with a very low impurity level.Further, the rate of reaction (7) is reduced by adding stannate to theelectrolyte and by keeping a low temperature. These are techniques beingwell known from the work of the inventor and others on alkalinealuminium/air and aluminium/oxygen batteries. On the other hand, it isdesired to operate at high temperature for reducing the polarization ofthe electrodes and to increase the conductivity of the electrolyte. Boththese factors increase the cell voltage under load.

Depending among other things on the load, how well the cathode iscatalyzed, the concentration of oxidant, the temperature and thealuminium alloy used, the typical cell voltage will be in the range of1.2 to 1.6 V for such a cell under load. Other factors affecting thecell voltage are ohmic losses in the system, given by the currentdensity, the geometry of the cell and the conductivity of the membrane.For keeping ohmic losses as low as possible, it is desired to minimizethe distance between the anode and cathode. Further, one desires to havelow resistance in the membrane separating the anolyte and catholyte. Fora given membrane material, the membrane resistance will decrease withdecreasing thickness, but, at the same time, losses of oxidant from thecatholyte to the anolyte increas with increasing losses according to (2)and (3) as a result. Further, mechanical strength sets a lower limit formembrane thickness. An elegant solution where a porous cathode acts asseparator between anolyte and catholyte is disclosed by C. L. Marsh etal in U.S. Pat. No. 5,445,905. Thereby the problem of voltage dropacross the membrane which separates anolyte and catholyte is avoided.For preventing contact between anode and cathode, a coarse-meshedinsulating net is utilized.

The advantages of a membrane-based system can be summarized by:

a) Low losses of capacity by direct reaction between oxidant and anode.

b) The concentration of oxidant can be kept high, which makes highcurrent densities on the cathode possible.

c) Solid particles in the anolyte (such as Al(OH)₃) does not affect thecathode reaction. while the disadvantages are

d) Complicated structure of the cells with separate circuits for anolyteand catholyte.

e) The membrane contributes to the ohmic losses in the battery.

f) Membranes are not very mechanically robust.

Especially for batteries for repeated use, the points d) and f) provideproblems with reliability. Such batteries are mechanically charged bydraining of the electrolyte, anodes are replaced, new electrolyte isfilled up and container with oxidant is refilled. This leads to demandsfor easy disassembling and leak proof operation which can be difficultto satisfy.

SUMMARY OF THE INVENTION

The above mentioned disadvantages of the prior art are solved byutilizing a cell for the production of electric energy by reactionbetween hydrogen peroxide or oxygen, and aluminium or lithium or amixture thereof, and hydroxyl ions in water, where the cathodes arecylindrical and based on radially oriented carbon fibres attached to astem of metal. The novel feature of the invention is that the anodes andcathodes are arranged in a flowing electrolyte of KOH or NaOH dissolvedin water, and where the electrolyte contains the oxidant in lowconcentration.

In the following, there will be given a detailed description withreference to the appended drawings which illustrate a preferredembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a cell based on separate anolyte andcatholyte.

FIG. 2 shows a schematic plan view of the cell and the circulationsystem.

FIG. 3 represents a vertical section of the battery cell.

FIG. 4 shows a perspective view of a p referred version of a cathode ofb rush shape.

FIG. 5 shows a graphical representation of the cell voltage as afunction of time during an experiment with a cell according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

FIG. 2 shows the main features of a cell 10 with anodes 1 and cathodes 2in an electrolyte 7. The cell produces electric energy by the reactionbetween hydrogen peroxide or oxygen, and aluminium or lithium or amixture thereof, and hydroxyl ions in water. Cathodes 2 are cylindricaland based on radially oriented carbon fibres attached to a stem ofmetal. Anodes 1 and cathodes 2 are arranged in a flowing electrolyte 7of KOH or NaOH dissolved in water, and with the electrolyte 7 containingthe oxidant in low concentration.

Pump means 6 is adapted to pump electrolyte 7 from an electrolyte outlet4 in the cell 10 to an electrolyte inlet 3 in the same cell. In apreferred embodiment, there is arranged an electrolyte distributor 5after inlet 3 on cell 10, which causes the electrolyte 7 to bedistributed as evenly as possible across the cross section of cell 10.

For a battery containing several cells, the electrolyte can move fromcell to cell (serial flow) or parallel via in-and outlet manifolds(parallel flow). Both cases can be based on the use of one or more pumpsfor the circulation and one or more pumps for the dosing of HP to theelectrolyte.

In a preferred embodiment of the invention the concentration of KOH orNaOH between 2 and 15 molar. The oxidant which is added to theelectrolyte is oxygen or hydrogen peroxide, in a preferred embodimentbetween 0.001 molar and 0.1 molar.

Anodes 1 and cell 10 are arranged for replacement of electrodes 1.Further, valves 20, 21 are arranged in the cell 10 for replacement ofthe electrolyte 7.

Anodes 1 and cathodes 2 are connected in parallel rows of their own.Cathodes 2 are in a preferred embodiment in the form of bottle brushes,as described in NO 171 937. FIG. 3 shows such cathodes mounted in a cell10, and FIG. 4 shows such a cathode separately.

On top of cell 10 there is mounted a one-way outwardly leading safetyvalve 24 arranged for ventilation of gas and for letting out surplus ofthe electrolyte 7.

The carbon fibres are catalyzed by a catalyst, which accelerates one orboth reactions, e.g. silver, platinum or palladium, but alsometal-organic cobalt-phtalocyanin orcobalt-tetrametoxy-phenylporphyrine.

During operation, a small surplus of HP is added to the electrolyte 7.By discharge at normal pressure, there has been used a stoichiometricfactor of 1.25, i.e. 25% surplus. At constant load this will after anadjustment period lead to a HP and oxygen concentration in theelectrolyte 7 which is given by the amount of HP added and the amount ofHP consumed by Faraday's law and by corrosion according to the equations(2) and (3) and by HP that decomposes to oxygen that is released to theenvironment. Typical value of HP in the electrolyte 7 is 0.003 M-0.005 Mabout 0.1 to 0.2 gram HP per litre electrolyte. For comparison, thepreferred concentration of HP in systems with separate catholyte andanolyte is about 0.5 M in U.S. Pat. No. 4,198,475 (Zaromb) and 5 M inU.S. Pat. No. 5,445,905 (Marsh).

An advantage with the present invention is that a battery can beproduced which has a high utilization of the reactants, combined withsimplicity, sturdiness and the possibility of quick mechanical charging.In this battery, losses due to reactions (2) and (3) are kept on a lowlevel as the concentration of oxidant is kept on a low level. For thesame reason, losses of oxidant because of oxygen emission to theenvironment are insignificant. Thus, catalysts may be used which notonly catalyze reaction (5), but also those which catalyze thedecomposition of H₂O₂ to oxygen (1) in combination with those whichcatalyze the reduction of oxygen to hydrogen peroxide:

O₂+H₂O+2e⁻=HO₂ ⁻+OH⁻  (8)

Reaction (8) is catalyzed by many low cost catalysts being described inthe literature on alkaline fuel cells. A catalyst of current interestfor reaction (8) is activated carbon, while the decomposition of HP iscatalyzed by most transition elements and mixtures thereof.

As cathodes, brush electrodes are applied as described in NorwegianPatent No. 171937. FIG. 4 shows a detail of a cathode. The cathode hasthe shape of a bottle brush with a core of metal and fibres of carbonfibre. The porosity is high: a cathode which is 10 cm long weighs about5 g and has a volume of 70 cm³. The electrode was originally developedfor a magnesium/oxygen sea water battery and is characterized by a highlimiting current even at low concentrations of oxidant and at a moderatethrough-flow of electrolyte. The conductivity of 7 M KOH isapproximately 30 times higher than sea water so that the current densityof this battery can be approximately 30 times higher than in a sea waterbattery with the same physical dimensions and at the same ohmic lossesin the electrolyte.

The carbon fibre surface is generally a poor catalyst, both with respectto the reduction of hydrogen peroxide and with respect to thedecomposition of hydrogen peroxide to oxygen. For this reason, thecarbon fibres are catalyzed with a catalyst that accelerates one or bothreactions. Among the candidates of current interest which is known fromthe literature on alkaline fuel cells, silver, platinum and palladiumare the most utilized, but also metal-organic compounds ascobalt-phtalocyanin and cobalt-tetrametoxy-phenylporphyrine areapplicable, especially after thermal heating of the carbon fibres to600-800° C. in an inert atmosphere.

The addition of oxidant in the form of HP leads to the total volume ofelectrolyte being increased with time, the contents of aluminate in theelectrolyte (according to (2)) being increased, and the concentration offree hydroxyl ions being reduced, with time.

Experience shows that as long as the concentration of aluminate is lessthan the concentration of free hydroxyl, the precipitation of aluminiumhydroxide, Al(OH)₃, will not take place until after a long time. This isin spite of the electrolyte being strongly supersaturated. Aprecipitation of aluminium hydroxide leads to an immediate increase inthe conductivity of the electrolyte because of the formation of freehydroxyl ions:

Al(OH)₄ ⁻=Al(OH)₃+OH⁻  (9)

For many aluminium-based battery systems, this is favourable because theelectrolyte is reformed, but in this system, the settlement of aluminiumhydroxide particles on the carbon fibres in the cathode leads to areduction in the cathode performance with time.

The demand for avoiding precipitation consequently determines thecapacity of the system, and will, together with the cell voltage,determine the energy density of the system.

For UUV applications, the system can be configured in two differentways. The system can have a constant mass or a constant electrolytevolume during discharge:

At a constant mass, the increase in electrolyte volume is received in anexpansion container, while at a constant electrolyte volume, there is aone-way valve between the cell/battery and the environment.

As HP is added, the surplus of electrolyte is let out to the sea via aone-way valve 24. By mounting one-way valve 24 to the highest point ofthe battery, the handling of hydrogen can simultaneously be attended to.With both configurations, the buoyancy of the battery is changed duringdischarge, which demands a system for active ballasting of the UUV. Thischange is the least for a system of constant mass. Further, a constantmass gives somewhat larger capacity in relation to electrolyte volume.On the other hand, a constant electrolyte volume gives a considerablesimpler system for gas handling (hydrogen) from the battery. Especiallyduring the ascent phase, where the electrolyte has been saturated withhydrogen under high pressure, there are considerable amounts of gas thathave to be released to the environment via a one-way or check valve 24.

So far, a considerable amount of experiments of the same cell design asshown in FIGS. 2 and 3 has been run. The anodes are 10 cm long massivecylinders of superpurity aluminium alloyed with 0.1% tin. The anodediameter is 25 mm. As cathodes we have utilized silver catalyzed carbonfibre brushes as shown in FIG. 4 with a nickel core and a diameter of 30mm. Experiments have been run both at atmospheric pressure and at apressure of 10 bar. Most of the experiments are made with cells whichhave 4 anodes and 10 cathodes, but cells in full scale for theapplication in ULTV's are tested.

For a preferred embodiment of a battery for UUV-application, a system isdesired where the battery can quickly be recharged after one and a halfday and night by changing electrolyte and filling HP up to three times,and which contains enough aluminium for operation for 100 hours atnominal load. For other applications a balanced system can be desired.

FIG. 5 shows voltage versus time for a four-anode cell at a load of 9 A.50% HP was added at 30% surplus. The start temperature was approximately20° C., and the cell was cooled in such a way that during theexperiment, the temperature was kept at 40° C. The electrolyte wascirculated through the cell at a rate of 0.3 litre/sec. As electrolyte,there was used 6 M KOH with 0.01 M Na₂SnO₃ added. The electrolyte volumein cell and piping was about 2.1 litre. After 48 hours discharge, theelectrolyte was replaced by new electrolyte. The experiment wasterminated after 94 hours after a total output of 847 Ah. Theconsumption of aluminium was 351 g, which indicates a corrosion of 19%.Based on the start area, this gives a corrosion current of 7,7 mA/cm².Stationary concentration of hydrogen peroxide in the electrolyte(determined by permanganate titration) was 2.8 mM.

Initial voltage rise is the result of the increase in the temperature ofthe electrolyte from room-temperature to 40° C., and thereafter the cellvoltage decreases as the conductivity of the electrolyte decreases.

By the above mentioned through-flow of 0.3 litre/sec, the pressure dropacross the cell was only about 1 cm water column, so that thehydrodynamic work the circulation pump has to perform is insignificant.

On the basis of the consumption of reactants, the specific energy forthe battery system is approximately 150 Wh/kg at a specific load of 3.3W/kg. By comparison, the best NiCd-batteries have a specific energy ofless than 50 Wh/kg. Taken into consideration that the weight of theAl/HP-battery submerged in sea water is less than 0.4 kg/litre, while aNiCd-battery weighs more than 1.3 kg/litre, this gives a UUV powered bya Al/HP-battery the possibility of carrying considerably larger amountsof energy than a UUV which uses NiCd-batteries in a pressure compensatedembodiment.

In comparison with conventional batteries in a pressure tank, theadvantage of using the Al/HP-battery is even greater. By running underincreased pressure, it is found that it is possible to reduce thesurplus of hydrogen peroxide from 30% down to 10% without affecting thecell voltage. This is the result of losses to the environment beingreduced because of the increased solubility of oxygen with increasingpressure. An additional increase in the energy density can be achievedby increasing the alkaline electrolyte concentration. By increasing theconcentration of KOH to 12 M, the energy density can be increased up toapproximately 250 Wh/kg in a balanced system.

No big difference results from utilizing NaOH instead of KOH in thebattery. The conductivity is somewhat lower, which gives a lower cellvoltage, but this is to some extent compensated for in that a NaOH-basedelectrolyte has a lower density, which entails reduced weight of thebattery in water.

It should also be added that a plate of aluminium can be used instead ofa row of cylindrical anodes, this without giving significant alterationsin the properties of the battery. The essential feature is that there isa system where material transport to the cathode is considerably easierthan material transport to the anode. For the battery of the example,this was achieved both by the active surface of the cathode beingconsiderably larger than the surface of the anode (8 m² versus approx.0.03 m²) and by the limiting current density by diffusion in acylindrical field towards the carbon fibre surface being considerablyquicker than the limiting current density by diffusion against an (inrelation to the thickness of the diffusion layer) anode surface which isplanar. Further, it was advantageous to have only one row of anodesarranged between two rows of cathodes as shown in FIGS. 2 and 3. Inaddition to increasing the difference between the anode and cathodearea, this gave an even consumption of the anodes during currentdelivery.

What is claimed is:
 1. A cell for the production of electric energy byreaction between hydrogen peroxide or oxygen, and aluminium or lithiumor a mixture thereof, and hydroxyl ions in water comprising: at leastone cylindrical cathode having radially oriented carbon fibres attachedto a stem of metal; at least one anode; and electrolyte, wherein theanode and the cathode are arranged in a flowing electrolyte of KOH orNaOH dissolved in water, and with the electrolyte containing oxidant inlow concentration range of 0.003 M-0.005 M.
 2. A cell according to claim1, wherein the concentration of KOH or NaOH is between 2 and 15 molar.3. A cell according to claim 1, wherein the oxidant is oxygen orhydrogen peroxide.
 4. A cell according to claim 1, wherein theconcentration of the oxidant is between 0.001 molar and 0.1 molar.
 5. Acell according to claim 1, further comprising a circulation pump whichforces the flowing electrolyte in a circuit from an electrolyte outlet(4) on the cell to an electrolyte inlet on the cell.
 6. A cell accordingto claim 5, further comprising an electrolyte distributor locatedimmediately after the inlet in the cell.
 7. A cell according to claim 1,wherein the electrodes and the cell are arranged for replacement of theelectrodes.
 8. A cell according to claim 1, wherein the cell furthercomprises valves arranged for replacement of the electrolyte.
 9. A cellaccording to claim 1, further comprising a one-way outwardly leadingsafety valve arranged for ventilation of gas and for letting out surplusof electrolyte.
 10. A cell according to claim 1, wherein the cell hasanodes connected in parallel and cathodes connected in parallel.
 11. Acell according to claim 5, wherein the circulation pump pumpselectrolyte from one cell into another cell.
 12. A cell according toclaim 5, wherein the cell further comprises valves arranged forreplacement of the electrolyte.
 13. A cell according to claim 6, furthercomprising a one-way outwardly leading safety valve arranged forventilation of gas and for letting out surplus of electrolyte.
 14. Acell according to claim 5, wherein the cell has anodes connected inparallel and cathodes connected in parallel.
 15. A cell according toclaim 8, wherein the cell has anodes connected in parallel and cathodesconnected in parallel.